A general second order complete active space self-consistent-field solver for large-scale systems

TL;DR

This paper introduces a new second order CAS-SCF solver capable of handling large active spaces and AO bases, improving convergence efficiency and flexibility for complex quantum chemical systems.

Contribution

A novel second order CAS-SCF implementation that decouples wavefunction and orbital optimization, enabling large-scale calculations with various active space solvers.

Findings

Successfully computed low-lying states of Fe(II)-porphine with large active space

Demonstrated efficiency with a DMRG solver in a CAS(22,27) active space

Achieved convergence using modest computational resources

Abstract

We present a new second order complete active space self-consistent field implementation to converge wavefunctions for both large active spaces and large atomic orbital (AO) bases. Our algorithm decouples the active space wavefunction solver from the orbital optimization in the microiterations, and thus may be easily combined with various modern active space solvers. We also introduce efficient approximate orbital gradient and Hessian updates, and step size determination. We demonstrate its capabilities by calculating the low-lying states of the Fe(\Roman{2})-porphine complex with modest resources using a density matrix renormalization group solver in a CAS(22,27) active space and a 3000 AO basis.